1. Field of the Invention
The invention relates generally to seals and more particularly to a seal for engaging a moving shaft.
2. Background Information
Many conventional seals and packings have relatively little capacity for wear. A particular problem which arises in certain environments e.g., in jet engine turbines or cryogenic vacuum pumps, is that extremes of temperature and/or pressure may render conventional lubricants impractical as a means of reducing the coefficient of friction between seal members and the shaft exterior. In such environments, flexible seals such as elastomers are unsuitable and therefore the provision of a seal having adequate wear capacity can pose a difficult problem.
Numerous prior patents address the problem of providing seals or packings capable of maintaining sealing contact as they wear. However, the wear capacity of the devices proposed by these patents is typically limited to a relatively small fraction of the shaft diameter, or require flexible components.
U.S. Pat. No. 391,991 describes a six-piece packing in which each piece is provided with a flange and one or more projecting side lips. Each of the side lips is engaged and pressed radially inward by a flange of an adjacent one of the six pieces. Radial gaps between the pieces enable them to move inward as they wear. Inward travel of the pieces is limited by the narrow width of the gaps. Once the pieces shift sufficiently to close the gaps and move into abutting relation with one another, the packing cannot compensate for further wear. Furthermore, the radial dimensions of the six pieces are small with respect to the shaft diameter. Accordingly, the wear capacity of this packing is quite limited.
U.S. Pat. No. 4,350,349 describes another six-piece rod packing which provides a pair of segmented rings. Each ring has radial gaps at its sealing surface. The gaps in the respective rings are offset from one another. As in the above-described U.S. Pat. No. 391,991, the wear life of the packing is limited by the widths of the gaps. It is also noteworthy that this configuration fails to provide 360.degree. sealing contact in any one plane.
Numerous other attempts have been made in the past to provide various improved features for packings. For example, U.S. Pat. No. 588,681 describes a steam packing for a piston rod which is intended to automatically adjust itself to unevenness in the wear of the piston rod or of the packing itself, and U.S. Pat. No. 3,305,241 describes an effort to provide an improved backing ring.
There remains a need for a shaft seal which has a relatively high capacity for wear and which is suitable for use in extreme environments in which lubrication and seal flexibility may be impractical.